This invention relates to the field of monitoring particles flowing in a stack.
When light interacts with particles, the particles may reflect, refract, diffract or absorb the light, the nature of the interaction depending on the size, refractive index and surface profile of the particles and the wavelength of the interacting light.
For objects that are small compared with the wavelength of the light, the light will undergo Rayleigh scattering and a proportion of the light may be redirected in all directions. As the object size increases such that it is comparable to the wavelength of the light a larger proportion of the scattered light is redirected in the forward direction within well-defined angular lobes. This phenomenon is known as forward light scattering or Mie scattering. As the object size increases still further such that it is much greater than the wavelength of light, classical geometric optics begin to dominate.
The light scattering approach to dust measurement brings with it problems associated with reliable discrimination between scattered light and stray light or residues from the incident beam. Particle monitoring systems are installed in many dirty processes and hence contamination of optical surfaces is an important issue. Similarly, calibration of existing monitoring systems does not optimally take into account the effects of contamination. Contamination of optical surfaces of the monitor itself may produce unwanted scattered light that is measured by the monitor, giving a false reading. As discussed above, particles of different size scatter light in different directions and so the amount of light received by a prior-art probe depends on the size of particles in the flow, as well as their number and mass density; that can lead to errors in measurement. Prior-art monitors use relatively short interaction lengths but short interaction lengths make measurements vulnerable to local inhomogeneities in the particle flow. Readings from prior-art monitors may be affected by the harsh conditions that are prevalent in many stacks. For example, probes using glass-fibres are unsuitable for operation above 350° C.-400° C. due to temperature limitations of the fibre cladding and fibres based on sapphire, which may be able to operate at those temperatures, are very expensive. A particular disadvantage of some prior art designs is that, although they provide mechanisms for checking the calibration of a probe, they do so by moving one or more parts of the system that is to be calibrated in a way that may cast doubt on the reliability of the calibration measurements.
International Patent Application No. PCT/GB2003/003073 (published as WO 2004/008117) describes a particle monitor for, monitoring particles flowing in a stack. A light source generates a measurement beam on a first side of the particle flow. The measurement beam is directed by an optical system towards the opposite side of the particle flow without the measurement beam being scattering from the particles. On that opposite side of the flow, a reflector reflects the measurement beam back towards the first side of the particle flow, via an optical system that directs the measurement beam into the particle flow. Light from the measurement beam is thereby scattered by the particles and the scattered light is detected by a detector on the first side of the particle flow. The instrument described in that International Patent Application has the advantage that it provides a contamination and calibration check that does not involve moving any parts of the system that is to be calibrated, which enhances the reliability of the calibration measurements. However, we have found that the instrument suffers from some significant problems.